Antimicrobial and antiparasitic activity of propyl-propane-thiosulfinate (PTS) and propyl-propane-thiosulfonate (PTSO) from Allium cepa against salmonid pathogens. In vitro and in vivo studies

The rapid expansion of aquaculture, particularly in salmonids such as Oncorhynchus mykiss (rainbow trout) and Salmo salar (Atlantic salmon) farming, has brought increased challenges related to bacterial, fungal, and parasitic infections. Traditional reliance on antibiotics and antiparasitic agents raises concerns about antimicrobial resistance and environmental impact, prompting the need for sustainable alternatives. This study explores the potential of Propyl-Propane-Thiosulfinate (PTS) and Propyl-Propane-Thiosulfonate (PTSO), sulfur compounds derived from Allium cepa , as antimicrobial and antiparasitic agents in salmonid aquaculture. The research combined both in vitro evaluations of PTS, PTSO and their combinations (PTS/PTSO) with in vivo trials to investigate their efficacy against key pathogens and parasites affecting both O. mykiss and S. salar . In vitro assays demonstrated significant activity against adult Caligus rogercresseyi parasites, as well as against microorganism such as Yersinia ruckeri, Aeromonas salmonicida, Lactococcus garvieae , Piscirickettsia salmonis and Saprolegnia parasitica , using methods such as disk-diffusion and Minimum Bactericidal/Fungicidal Concentrations (MBC/MFC). Antiparasitic activity was assessed through tests on C. rogercresseyi in S. salar , where PTS/PTSO combinations significantly reduced parasite viability, particularly during early developmental stages. Complementing these findings, in vivo trials tested the effects of PTS/PTSO-supplemented diets on rainbow trout, demonstrating enhanced survival rates and reduced disease burdens under controlled challenges. Despite challenges related to compound stability and delivery within aquaculture systems, PTSO consistently outperformed PTS, showcasing superior antimicrobial and antiparasitic efficacy. These results highlight the potential of organosulfur compounds from onion as innovative solutions for disease management in salmonid aquaculture. • PTS and PTSO from Allium cepa show strong activity against salmonid pathogens in vitro . • PTS/PTSO-supplemented diets improve survival rates in rainbow trout under pathogen challenges. • Salmo salar fed with PTS/PTSO-supplemented diets exhibit reduced Caligus rogercresseyi loads. • PTSO demonstrates superior antibacterial, antifungal, and antiparasitic efficacy compared to PTS. • Organosulfur compounds offer sustainable alternatives to antibiotics in salmonid aquaculture.